Is this considered cheating?

Wow...I applaud you. I would have never thought to change the phone number to mine. You are a genius! But, to answer your question...if he's willing to text her those things, the boundary has already been broken, There is nothing stopping him from acting on his desires. He only gave you the line about "thinking and doing are two different things" because HE GOT CAUGHT!!!! I definitely think you have a real problem on your hands.